This invention relates to electrochemical sensors. More particularly, this invention relates to sensors for determination of electrochemical corrosion potential (ECP) of metal components in liquids at high temperatures and pressures. This invention also relates to methods for making electrochemical corrosion potential sensors.
Many areas of industry, such as, for example, the power generation industry, employ metal structural components that are exposed to liquids at high temperatures and pressures. Examples of systems in which equipment is designed for such exposure include nuclear reactors, including the boiling water and pressurized water reactor types; fossil fuel systems; and geothermal systems. In a boiling water nuclear reactor, for example, water and steam are channeled through various conduits formed of stainless steel. Normal water chemistry conditions include high oxidizing species, such as oxygen and hydrogen peroxide, which may lead to intergranular stress corrosion cracking (IGSCC) of the stainless steel.
IGSCC can be mitigated by lowering the concentrations of ionic impurities and oxidizing species in the reactor water. In a nuclear reactor, for example, this lowering of impurity concentration may be effected using hydrogen water chemistry (HWC) in which hydrogen is added to the feed water of the reactor. The primary purpose of the added hydrogen is to reduce the dissolved oxidant concentrations and thereby lower the ECP below a critical value at which IGSCC susceptibility is significantly reduced.
Various forms of ECP sensors are used for measuring ECPs in nuclear reactors and other systems. The sensors have different configurations for measuring ECPs, and are subject to different problems that limit their useful lives. For a nuclear reactor, for example, the useful life of a sensor should cover the duration of at least a single fuel cycle, which is typically in the range from about 18 months to about 24 months in the United States. However, experience in actual nuclear reactors has demonstrated sensor failure in a shorter duration due to various causes.
One type of ECP sensor, disclosed in U.S. patent application Ser. No. 09/397,840, now U.S. Pat. No. 6,370,213 commonly owned by the present assignee, includes a ceramic probe in which is packed a mixture of metal and metal oxide powder for providing a corresponding reference ECP. This mixture may include iron and iron oxide (Fe/Fe3O4), or copper and copper oxide (Cu/Cu2O), or nickel and nickel oxide (Ni/NiO).
In this type of sensor, the probe is typically in the form of a zirconia tube brazed to a support tube made of a suitable metal such as INVAR(trademark) low-expansion alloy with a nominal composition of 36% by weight nickel, 64% by weight iron, and  less than 1% other additions, or other alloy with a suitably low thermal expansion such as, for example, alloy 42, with a nominal composition of 42% by weight nickel, balance iron. This support tube in turn is often welded to a stainless steel tube. An electrical conductor extends through the tubes into the probe and is buried in the operative mixture.
In one example, the ceramic probe is formed of magnesia-stabilized-zirconia (MSZ) brazed to an alloy 42 support tube. Since the ceramic probe and metal tube have different coefficients of thermal expansion, they are subject to thermal shock during high temperature operation which can lead to cracking of the braze joint.
The braze material is also subject to corrosion during operation. Both problems potentially limit the useful life of the sensor, because failure of the braze joint causes water leakage inside the sensor and failure thereof.
To address these problems, the aforementioned U.S. patent application Ser. No. 09/397,840 discloses and claims an ECP sensor designed to mitigate degradation of this braze joint by employing a ceramic band, often applied via plasma spraying, selectively applied around the perimeter of the sensor for bridging the probe and support tube at the braze joint for covering and sealing thereof. Although such a band provides improved protection for the sensor, plasma sprayed ceramics may occasionally exhibit undesirable levels of porosity, which may affect the ability of the ceramic to provide a barrier against corrosion.
Accordingly, it is desired to provide an ECP sensor with improvements addressing these problems.
Embodiments of the present invention are provided to address the issue of ECP sensor reliability. One embodiment provides a sensor for measuring electrochemical corrosion potential comprising: a tubular ceramic probe having a closed tip at one end, the probe at least partially filled with a powder comprising metal and metal oxide; a metal support tube having one end receiving an opposite end of the probe, and joined thereto by a braze joint therewith; an electrical conductor extending through the support tube and into the probe, and having an end buried in the powder for electrical contact therewith; and a protective band bridging the probe and tube at the joint for sealing thereof, the protective band consisting essentially of a metallic coating. A second embodiment provides a method for manufacturing a sensor for measuring electrochemical corrosion potential, the method comprising: providing a tubular ceramic probe having a closed tip at one end, the probe at least partially filled with a powder comprising metal and metal oxide; providing a metal support tube having one end receiving an opposite end of the probe; joining the tube with the probe by forming a braze joint therewith; and depositing a protective band bridging the probe and tube at the joint for sealing thereof, the protective band consisting essentially of a metallic coating.